Multi-leaf collimators (MLC) are used (principally) in the field of radiotherapy. A beam of radiation is directed toward a patient and must be collimated to fit the shape of the area to be treated. It is important to ensure that the dose in the areas outside that shape is as low as possible, but also that the whole area is treated. If areas are left untreated then the likelihood of recurrence is increased, whereas if non-treatment regions are irradiated then damage will be caused to healthy tissue resulting in greater side effects and longer recovery times after treatment.
As the treatment area is rarely rectilinear, multi-leaf collimators are employed. These comprise an array of finger-shaped leaves of a radiation-absorbing material, each disposed in a parallel relationship and each able to move longitudinally relative to the others. By moving each leaf to a selected position, a collimator is provided which can exhibit a non-linear edge. In general, one such array (or “bank”) will be provided on each side of the beam.
Previously, the leaves have been driven by various means. One involves a threaded shaft extending rearwardly away from the leaf; this can be supported in a threaded bore or connected to the leaf via a threaded socket. In either case, as the shaft or bore is rotated the leaf will be forced to move. An example is shown at U.S. Pat. No. 4,868,844 in which motors are placed to one side and linked to the threaded shafts by a flexible shaft. Activation of the motor under microprocessor control forces the threaded shaft to rotate and move through the threaded bore in which it is held. This then urges the leaf in the appropriate direction. The use of flexible shafts allows the motors to be spatially separated from the leaves and allows for the facts that the motors are significantly wider than the leaves.
A further example is shown in FIG. 9. The leaf 200 is supported on its lower edge by a roller bearing 202, and is guided along its upper edge by a pair of roller bearings 204, 206. A longitudinal slot 208 is cut into the leaf, extending from the rear edge 210 towards the front edge 212. At the start of the slot 208, proximate the rear edge 210, a threaded nut 214 is fixed in place, in line with the slot. A leadscrew 216 is then threaded through the nut 214 and sits in the slot 208.
A motor and gearbox 218 are positioned behind the leaf and drive the leadscrew 216 via a shaft support and coupling 220. Thus, as the leadscrew is driven, the nut 214 and hence the leaf 200 will be driven rearwardly or forwardly, depending on the direction of rotation.
Other designs use a rack and pinion system, where a toothed rack is cut into the edges of the leaves, and motors are mounted outboard of the leaves with a drive shaft that extends perpendicular to the leaves, across the bank. Each shaft carries a pinion at the relevant location so as to engage with the rack of the appropriate leaf.
Multi leaf collimators are now being designed with smaller resolution leaves which are therefore thinner and more numerous. A significant problem in doing so is the need to drive the leaves, i.e. provide a means of physically moving them to the required degree of accuracy, and the impact of this on the length of a leafbank, the length of a leaf, and its the complexity and serviceability. This in turn increases the size of the treatment head and can restrict the patient treatment access.
If the leaves are driven from one end, then the motors are outboard and undesirably add length to the assembly. Likewise, reductions in the leaf width mean that it becomes increasingly difficult to embed a leadscrew.
The existing rack & pinion methods also become increasingly difficult with a larger number of leaves, and usually result in extending the leaf length in order to accommodate the necessary number of motor drives.